This invention relates to television and data display terminals, and more particularly to a circuit for sinusoidal modulation of the rasters at a predetermined rate.
To enhance the display of video or alphanumeric data on a cathode ray tube (CRT), it is desirable to synchronously modulate the raster scan at a predetermined frequency with an amplitude sufficient to significantly span the space between the rasters. For video, 1/2 raster spacing may be sufficient, but for data display, the amplitude of modulation may be greater. In either case, it is desirable to introduce the modulation with an efficient circuit.
For the purpose of illustrating the invention, reference will be made to data display terminals, since application of the modulation circuit to a television receiver or monitor will be analogous. The raster scan patterns differ only in the number of rasters per frame, and in the fact that for television the rasters are divided into two interlaced fields. In every case, the horizontal sweep circuit is reest to zero, and as the horizontal sweep increases for one raster, the vertical sweep increases for the amount of the desired raster spacing, either in one initial step, or more commonly for television receivers and monitors, at a low constant rate.
In a typical data display terminal, a frame is divided into 80 columns and 24 rows. Each column provides a character space, and each row provides a line of characters. The character space defined by a column and row count is further subdivided into a matrix of dot positions, typically 8.times.11, where each of seven horizontal dot positions in each of ten rasters (scan lines) may be selectively brightened to make up a character. The useful dot matrix within a character space is thus 7.times.10, leaving a clear raster to separate lines of characters, and a clear column at the end (or beginning) of each character to separate characters in a line. Consequently, the entire field displayed is divided into an array of 560.times.264 adjacent dot spaces, even though some spaces are not used, to provide spaces between characters and lines of characters, and within a 7.times.10 matrix, only those actually needed to form a character are used while displaying data.
For data display purposes, a clock generator operating in the megahertz range is divided down to obtain a 60 Hz vertical (V) sync rate, and down further to get horizontal (H) sync rates, thereby producing a field display at the rate of 60 per second. This chain of dividers will not only synchronize the data display with the horizontal and vertical scan of noninterlaced fields, but provide the addressing information necessary to read out into a shift register trains of binary digits, where each bit 1 will cause the beam to brighten a dot as a raster is scanned. When the entire set of rasters for a line of characters have been scanned, and all 24 lines of data have been displayed, the data stored in a RAM will have been displayed in 80.times.24 character spaces.
For each character space, the shift register is loaded with a new train of binary digits as a line of data is displayed. These binary digits define the dots to be displayed and, as the last of the previous train is shifted out into a video mixer that combines sync and blanking with the binary digits into a composite signal for display, the next set of binary digits is loaded into the shift register. In the CRT display unit, a horizontal (H) and vertical (V) drive generator responds to the horizontal and vertical sync pulses to produce the horizontal and vertical drive signals applied to deflection coils, while the binary digits from the shift register, and the blanking signals, are applied to the cathode of the CRT. In that way, the beam is brightened for dots defined by 1 bits out of the shift register, and blanked at all other times while 0 bits are shifted out and while the blanking signals for raster and field retrace intervals are present.
To form a line of characters the clock frequency divider is used to address a random access memory (RAM) for each line of 80 characters, one character at a time in sequence. Each output character code, together with the output of a counter that counts the lines of characters, addresses a character generator implemented with a read only memory (ROM) to produce in sequence the corresponding lines of binary digits that define the characters in the row addressed. A shift register receives the binary digits in parallel for one character at a time in sequence, and converts them into a continuous serial train. After the procedure has been repeated ten times for one line of 80 characters, the address to the RAM is advanced to the next line of 80 characters. In that manner the output of the RAM addresses the character generator to convert the character code out of the ROM into the binary digits that define the positions of dots for the characters.
As described in a copending application Ser. No. 686,219 filed Dec. 26, 1984, display is enhanced by vertical modulation of the horizontal raster scans at a frequency that will produce one complete cycle per Mxn dot matrix space. For optimum results, the depth of modulation should be at least .+-.1/4 the spacing of the raster scans, depending on beam width, dot duration, and line spacing. Then, as an M-bit code for a character to be displayed is read out of a shift register, clocked at the frequency required for a line of data to be displayed in a raster scan as a dot for every bit 1 in the M-bit code, each dot is displayed during a portion of one cycle of modulation in the dot space.
The phase of the modulation is adjusted relative to the shift register clock so that a dot is displayed while the beam is being deflected between negative and positive maxima, on either the positive or the negative slope. In that manner a fixed Mxn dot matrix space is used for each character with the elliptical dots displaced with their major axis at an angle with respect to the horizontal. The space between dots in a direction having a vertical vector component is thus reduced, and in the special case of the direction being about half way between the horizontal and the vertical, such as at 45.degree., a nearly perfect continuous line will appear. This improvement is achieved without significantly degrading the horizontal resolution. This is because the dot spacing is not as great in the horizontal direction as in the vertical. i.e., the dot spaces are usually rectangules having a width that is less than the height. This is particularly true of the state-of-the-art 80 column display, as opposed to a 40 column display. The more noticeable effect is a thickening of horizontal portions of the characters displayed with some lessening of thickness in the portions having a vector component in the vertical direction, and near perfect portions having a slope of about +45.degree. if the dots are displayed on the positive slope of the modulation, or a slope of about - 45.degree. if the dots are displayed on the negative slope of the modulation, but not both; in one there will still be some space between dots discernable, but in the more usual dot-matrix, alphanumeric characters, only a small percentage of the character lines are in the disadvantageous direction. Even in upper case letters, about one out of five will have not more than about 20% of their lines at the disadvantageous direction, so the net effect is a significant improvement in character resolution in about 96% of an 80.times.24 character display. If the dot is displayed at the peak (positive or negative) of the modulation, a crescent shaped dot will be produced that will tend to fill vertical gaps between the rows of dots.